The human auditory system, including related brain functions, is capable of localizing sounds in three dimensions notwithstanding that only two sound inputs are received (left and right ear). Research over the years has shown that localization in azimuth, elevation and range is dependent on a number of cues derived from the received sound. The nature of these cues is outlined below.
Azimuth Cues—The main azimuth cues are Interaural Time Difference (ITD—sound on the right of a hearer arrives in the right ear first) and Interaural Intensity Difference (IID—sound on the right appears louder in the right ear). ITD and IIT cues are complementary inasmuch as the former works better at low frequencies and the latter better at high frequencies.
Elevation Cues—The primary cue for elevation depends on the acoustic properties of the outer ear or pinna. In particular, there is an elevation-dependent frequency notch in the response of the ear, the notch frequency usually being in the range 6–16 kHz depending on the shape of the hearer's pinna. The human brain can therefore derive elevation information based on the strength of the received sound at the pinna notch frequency, having regard to the expected signal strength relative to the other sound frequencies being received.
Range Cues—These include:                loudness (the nearer the source, the louder it will be; however, to be useful, something must be known or assumed about the source characteristics),        motion parallax (change in source azimuth in response to head movement is range dependent), and        ratio of direct to reverberant sound (the fall-off in energy reaching the ear as range increases is less for reverberant sound than direct sound so that the ratio will be large for nearby sources and small for more distant sources).        
It may also be noted that in order avoid source-localization errors arising from sound reflections, humans localize sound sources on the basis of sounds that reach the ears first (an exception is where the direct/reverberant ratio is used for range determination).
Getting a sound system (sound producing apparatus) to output sounds that will be localized by a hearer to desired locations, is not a straight-forward task and generally requires an understanding of the foregoing cues. Simple stereo sound systems with left and right speakers or headphones can readily simulate sound sources at different azimuth positions; however, adding variations in range and elevation is much more complex. One known approach to producing a 3D audio field that is often used in cinemas and theatres, is to use many loudspeakers situated around the listener (in practice, it is possible to use one large speaker for the low frequency content and many small speakers for the high-frequency content, as the auditory system will tend to localize on the basis of the high frequency component, this effect being known as the Franssen effect). Such many-speaker systems are not, however, practical for most situations.
For sound sources that have a fixed presentation (non-interactive), it is possible to produce convincing 3D audio through headphones simply by recording the sounds that would be heard at left and right eardrums were the hearer actually present. Such recordings, known as binaural recordings, have certain disadvantages including the need for headphones, the lack of interactive controllability of the source location, and unreliable elevation effects due to the variation in pinna shapes between different hearers.
To enable a sound source to be variably positioned in a 3D audio field, a number of systems have evolved that are based on a transfer function relating source sound pressures to ear drum sound pressures. This transfer function is known as the Head Related Transfer Function (HRTF) and the associated impulse response, as the Head Related Impulse Response (HRIR). If the HRTF is known for the left and right ears, binaural signals can be synthesized from a monaural source. By storing measured HRTF (or HRIR) values for various source locations, the location of a source can be interactively varied simply by choosing and applying the appropriate stored values to the sound source to produce left and right channel outputs. A number of commercial 3D audio systems exist utilizing this principle. Rather than storing values, the HRTF can be modeled but this requires considerably more processing power.
The generation of binaural signals as described above is directly applicable to headphone systems. However, the situation is more complex where stereo loudspeakers are used for sound output because sound from both speakers can reach both ears. In one solution, the transfer functions between each speaker and each ear are additionally derived and used to try to cancel out cross-talk from the left speaker to the right ear and from the right speaker to the left ear.
Other approaches to those outlined above for the generation of 3D audio fields are also possible as will be appreciated by persons skilled in the art. Regardless of the method of generation of the audio field, most 3D audio systems are, in practice, generally effective in achieving azimuth positioning but less effective for elevation and range. However, in many applications this is not a particular problem since azimuth positioning is normally the most important. As a result, systems for the generation of audio fields giving the perception of physically separated sound sources range from full 3D systems, through two dimensional systems (giving, for example, azimuth and elevation position variation), to one-dimensional systems typically giving only azimuth position variation (such as a standard stereo sound system). Clearly, 2D and particularly 1D systems are technically less complex than 3D systems as illustrated by the fact that stereo sound systems have been around for very many years.
In terms of user experience, headphone-based systems are inherently “head stabilized”—that is, the generated audio field rotates with the head and thus the position of each sound source appears stable with respect to the user's head. In contrast, loudspeaker-based systems are inherently “world stabilized” with the generated audio field remaining fixed as the user rotates their head, each sound source appearing to keep its absolute position when the hearer's head is turned. In fact, it is possible to make headphone-based systems “world stabilized” or loudspeaker-based systems “head stabilized” by using head-tracker apparatus to sense head rotation relative to a fixed frame of reference and feed corresponding signals to the audio field generation system, these signals being used to modify the sound source positions to achieve the desired effect. A third type of stabilization is also sometimes used in which the audio field rotates with the user's body rather than with their head so that a user can vary the perceived positions of the sound sources by rotating their head; such “body stabilized” systems can be achieved, for example, by using a loudspeaker-based system with small loudspeakers mounted on the user's upper body or by a headphone—based system used in conjunction with head tracker apparatus sensing head rotation relative to the user's body.
As regards the purpose of the generated audio field, this is frequently used to provide a complete user experience either alone or in conjunction with other artificially-generated sensory inputs. For example, the audio field may be associated with a computer game or other artificial environment of varying degree of user immersion (including total sensory immersion). As another example, the audio field may be generated by an audio browser operative to represent page structure by spatial location.
Alternatively, the audio field maybe used to supplement a user's real world experience by providing sound cues and information relevant to the user's current real-world situation. In this context, the audio field is providing a level of “augmented reality”.
It is an object of the present invention to facilitate clear presentation in audio user interfaces in which sound sources are synthesised in an audio field.